Figure 3. γ-COP and β-COP appendage domains within the coat.
(A) Localization of the γ-COP and β-COP appendages in the COPI leaf. The 9.2 Å EM map of the COPI leaf is colored based on the underlying subunit. Color scheme as in Figure 1, additionally appendage sandwich subdomains are red and appendage platform subdomains are purple. (B) γ-COP appendage (sandwich subdomain – red, platform subdomain - purple) interacts with the β’-COP (light blue). The models are shown within the corresponding part of the COPI EM map (transparent grey isosurface) (C) The structure as in B, opened out to reveal the conserved interaction interfaces of γ-COP and β’-COP (purple – conserved, cyan - variable). The yellow asterisks mark the interaction surfaces. (D) β-COP appendage interacts with α-COP (dark blue). (E) The structure as in D, opened out to reveal the conserved interaction interfaces of β-COP appendage and α-COP (purple – conserved, cyan - variable). (F) The β-COP appendage domain provides the main structural connection between the γ-COP adaptor subunit (light green) and the outer-coat subunit α-COP (blue), where it interacts with both β-propeller domains. Organization of the β-COP subunit: the long β-COP trunk domain (dark green) is connected to the appendage domain (red/purple) by a flexible linker (dashed orange line). We note that the β-COP trunk domain conformation, which is rather straight, is significantly different from the conformations of the β-subunits from homologous AP complexes, which are highly curved. The panel on the right shows the subcomplex from the ‘bottom’ (from the membrane side) in order to visualize the suggested β-linker path more clearly.